The Arcane Realm is a realm full of infinite energy. All spells cast by the Arcanists are using the resources located within this plane. The mana in the blood of the Arcanists possesses the ability to call forth the arcane energy and manipulate it to their special will. In essence, a stream of fire called forth by an Evoker is using flame energy from this realm.
The Arcane Realm is not technically a physical place. It cannot be traveled to, though it simply just ‘is’.